Zork II

"Expect the unexpected when you take on ZORK II: THE WIZARD OF FROBOZZ. As you explore the subterranean realm of Zork, you'll continually be confronted with new surprises. Chief amongst these is the Wizard himself, who'll endeavor to confound you with his capricious powers. But more than that, you'll face a challenge the likes of which you've never experienced before."

In version 7, you were able to give commands to some inanimate objects. Contrary to the report in XYZZYnews #2, you cannot use the standard Infocom command syntax (such as AQUARIUM, GO EAST, which generates the reply "You must supply a verb"), but must use the same types of commands that you use to direct the robot. For example, you could say TELL AQUARIUM "GO EAST", and the game would say "The aquarium has left the room." TELL AQUARIUM "GO EAST THEN GO EAST" results in a message saying that you have been killed by a grue, but in reality it is the aquarium that has "died.", as the next line informs you.
There is a parser bug in Versions 7, 17, 18, 19, 22, and 23 that stems from having two objects of the same color in the same room (such as the blue label and the blue sphere), and then referring to the object simply by its color when trying to take it. For example:

>TAKE THE BLUE
Which do you mean?

In the LTOI version (48/840904) the parser bug has been fixed, and the reply "There seems to be a noun missing in that sentence" replaces the garbage. (Early parser versions of other Infocom games may have similar bugs.)
In versions 7, 17, 18, 19, 22, and 23, you could take immovable objects by reading them. For instance, "READ UNICORN" will (provided the unicorn is actually present in the first place) reply:

Taken. How can you read a unicorn?

Another example: Reading the stands on the Wizard's workbench will let you take them, even though the game claims they are firmly bolted to the table. However, since the text about the stands is a part of the room description, they remain attached to the table, even if you're carrying them, and even if you drop them elsewhere, even though the table itself doesn't move along with them.
When the wizard appears before you, but doesn't cast any spell, you will see the following text:

The Wizard seems about to say something, but thinks better of it, and
peers at you from under his bushy eyebrows.

If you type "KILL THE WIZARD," you will get the following reply:

The Wizard retreats, waving his wand and chanting. He says, "Fear!"
Nothing happens! With a terrified glance at the demon, the Wizard runs
past you and out of the room.

This text clearly belongs much later in the game, after you've freed the demon and ordered him to kill the wizard. This works in all versions.
In all versions, if your sword is "filched" by the Wizard, filching it back with the Wizard's wand may be a little confusing, since there are nicked swords of other adventurers in the room also. If you try to "point wand at sword", you will be asked "Which sword do you mean, the sword, or the nicked swords?" Saying "the sword" will not work and starts the process over again. Fortunately, this can be gotten around if you have the sense to say "the Elvish sword". -- Graeme Cree
Yet another early Zork II bug: If you looked into one of the three spheres when they were all in the same place, the game would recursively describe the room as seen through each sphere, which would overflow the interpreter stack.

NOTE: I couldn't get this to work in either Version 7 or 17. If I looked into a sphere while holding all three, I saw only darkness. If I dropped them all and looked into one, the room was described normally, with no crash. However, one of the two spheres that I didn't look into was always missing from the room description. -- GC
Another minor parser bug: Restart the game, and try using a pronoun in your first command, such as "EXAMINE IT." You should get the reply:

There's nothing special about the .

This happens in other games running early versions of the Infocom parser, but I first noticed it with Zork II. In games where the problem has been corrected, it will say "There's nothing special about the ."

NOTE: In all versions, I got "There's nothing special about the ." --GC
If you time it right, you can blow up the base of the volcano while in the balloon. This requires dropping the bomb on the ground and lighting it, then crawling into the basket before it explodes. If you do it right, the computer should tell you that the balloon settles to the ground, followed by "You can't go that way." -- John Payson